In large animal models of hypoxic pulmonary hypertension (PHTN) that closely parallel human disease, the earliest pulmonary artery (PA) smooth muscle cell (SMC) proliferative changes occur at the medial/adventitial border. Migration of SMC and/or myofibroblasts to more distal vessels is also a prominent feature. A murine model that adequately mimics these hypoxia-induced changes has not been described. Neutral endopeptidase (NEP; or neprilysin) is an important cell surface peptidase that degrades vasoactive neuropeptides (like the bombesin-like peptides [BLPs]), that may promote vascular remodeling. NEP has also been shown to directly engage in intracellular signaling by novel peptidase independent mechanisms. Finally, NEP has recently been associated with decreased inflammation, carcinogenesis and growth. These observations support the possibility that in the lung (in contrast to the heart and systemic vasculature) NEP could exert a protective effect on susceptibility to hypoxic PHTN. We now have strong preliminary evidence to support this concept. We have found that targeted deletion of NEP in mice predisposes to exaggerated hypoxic PHTN. The resulting structural changes are more substantial than in previously described mouse models and for the first time demonstrate proximal changes at the medial/adventitial border. Recruitment of dedifferentiated SMC or myofibroblasts into the distal circulation is also a major feature. This unique pattern of vascular remodeling, together with intriguing observations in the literature, suggest key roles for BLPs, BLP receptors, selected isozymes of protein kinase C (PKC alpha, delta, and epsilon), Rho, and focal adhesion kinase (FAK). The following hypotheses will be tested: #1) NEP protects the lung vasculature from the development of hypoxic PHTN and limits vascular remodeling by suppressing the proliferation, migration, and contraction of PA SMC. #2) Selected neuropeptides (initial focus: BLPs) are largely responsible for the exaggerated pulmonary vascular remodeling observed in the chronically hypoxic NEP knockout (KO) mouse. Hypoxia- induced upregulation of BLPs and BLP receptors contributes to the increased medial/adventitial changes. #3) Upregulation of BLP post-receptor signaling intermediates (PKC a, delta , and epsilon, Rho, and FAK) contributes to the exaggerated remodeling. NEP inhibits these signaling intermediates as well as proliferation and migratory responses of PA SMC by peptidase dependent and independent mechanisms. Integrated experiments will be performed in single and double KO mice, perfused lungs, isolated pulmonary arteries and PA SMC. These studies will draw on a unique mouse model of hypoxia-induced pulmonary vascular remodeling to increase our understanding of the mechanisms that control susceptibility to hypoxic-PHTN and could identify new therapeutic targets to limit or reverse this important clinical problem.